As he emerged from the dark, looted interior, John Curtis, of the British Museum, paused briefly before agreeing that perhaps not since the Second World War had there been such a comprehensive theft and destruction of a major museum. Up to 20 priceless statues and other objects have been either smashed or stolen from the main galleries, with countless more artefacts taken from one of the two basement storehouses.

Dr Curtis, keeper of the Department of the Ancient Near East, had just travelled 2,500 miles from London to spend four hours inside a museum where he could record details only of decapitated statues, smashed artifacts and empty plinths. He will make the return journey today to present an urgent report to a British Museum and Unesco meeting in London on Tuesday.

In the courtyard beneath the blue dome of the museum, he outlined the extent of the destruction wrought in the offices, which contain the catalogue and archives, the exhibition galleries and the storehouse.

“Heartbreaking, tragic,” he said, shaking his head. “A great loss to the cultural heritage of the world, not just Iraq. All of the objects had been taken out of the galleries before the war, except the very heavy things that couldn’t be removed. They are completely irreplaceable. I saw evidence of about 20 things that have been dragged away and removed. You can see this because the treads on all the stairs have been broken.”

Among the missing objects: an Akkadian bronze statue, dating to 2400BC, of King Naram-Sin; an Islamic wooden door; the head of a Roman emperor, perhaps Trajan; and an Assyrian stone statue of King Shalmaneser III, circa 800BC.

But perhaps the most significant theft is of a 4ft-high decorated Sumerian stone temple vase found at Warka, near Uruk, dating from 3500BC. “It is one of the very few examples of Sumerian figural art,” Dr Curtis said. “It is the only one of its kind on that scale. The problem is we don’t know anything about what has happened in other places around Iraq. The preliminary news from Mosul is very bad, that the museum has been completely emptied out.”

The Shalmaneser statue was, in fact, handed back earlier this week, smashed into six pieces, by one of the many nervous Iraqis filing back to the museum gates with bags full of pottery and fragments after the declaration of an unofficial amnesty.

“We didn’t ask him about the statue. We can repair it. Very fine people, and faithful,” Jaber Khalil, chairman of the State Board of Antiquities and Heritage, said, with more diplomacy than conviction.

Forty-six items were returned yesterday alone, but experts fear that many, too recognisable to be sold on the open market, are headed for unscrupulous private collectors in New York and Europe.

Iraqi and American investigators are convinced that some of the looters knew exactly what they were looking for, and may even have been looting to order. As he emerged from the museum, Donny George, of the State Board of Antiquities and Heritage, explained what aroused their suspicions. “We found these inside,” he said, holding a fistful of glass-cutters collected from the exhibition galleries. “And they passed by pieces made from gypsum because they knew what they wanted.”

He said that the museum had made preparations for such an eventuality years earlier by removing valuables to places of safety, such as the Central Bank of Iraq — to which US investigators have yet to gain access — and shifted smaller items into the basement in the weeks leading up to the war. But they did not expect the museum to be left unguarded after the security apparatus had fled.

Mr George is clearly angry, claiming that the Americans left the museum unguarded for days. He is not the only one. Repeating an Iraqi conspiracy theory so universal that it is now received wisdom, Mohammed Sabri, a prehistoric specialist, said bitterly: “They were late. They should have been here from the first day. The Ministry of Oil was protected the first day, why not the museum. Why? Ask the Americans.”

The Americans have a ready answer. “We were fighting the whole time,” Captain Jason Conroy said, wiping his brow as he guarded the museum gate. “For four days we were taking machinegun-fire and rocket-propelled grenades (RPGs) from these buildings around here. They had a bunker around the back of the museum with a cache of RPGs. Guys were running out of that alley, firing Kalashnikovs at us.

When we shot them, they threw out hooks, dragged the bodies and guns back and came at us again.”

After four days of intense street battles with Saddam’s Fedayin and Special Republican Guards, Captain Conroy said, his company of Abrams tanks and armoured vehicles was ordered north on April 15 to destroy an anti-aircraft gun.

“When we got back the next day, everything was already on fire here and the press were here asking us: ‘How come you weren’t in the museum three days ago?’ I said: ‘If you guys had been here three days ago, you would know why.’”

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